Poly(ADP-ribose) polymerase or poly(ADP-ribose) synthase (PARS) is one of the enzymes (EC 2.4.2.30) which catalyse the transfer of ADP-ribose from NAD to the carboxyl groups of proteins. It is mainly found in the cell nucleus and for many years it has been considered that its activity is directed towards the maintenance of the integrity of the DNA sequence by in some way participating in the repair of damage undergone by the gene sequence after toxic or infective insults. Some histones, topoisomerase I and II, DNA ligase and other nuclear proteins are substrates of this enzyme. Moreover, PARP itself can undergo self-poly-ADP ribosylation.
The cDNA which codes for PARP has been cloned and the protein has been purified from tissues of many animal species, including man. The structure of PARP is available in various data banks, and consists of:                1. an amino terminal domain capable of binding DNA (42 kDa),        2. a central zone also called self-modification domain (which thus can be ADP-ribosylated and which on average weighs 16 kDa), and        3. a carboxy terminal domain which contains the catalytic site (55 kDa) and which has an amino acid composition and a tertiary structure highly conserved from drosophila to man.        
The necessary and sufficient stimulus for activating the enzyme is damage to the DNA (formation of “nicks” or other lesions of the double helix) (de Murcia et al., 1992; Menisser-de Murcia et al., 1997). This commonly occurs by the action of strong oxidants and/or of highly reactive molecules such as the free radicals which form abundantly in the tissues in the course of various pathological situations. Nitric oxide can also directly or indirectly damage the DNA double helix, activate PARP and lead the cell to massive utilization of NAD and to ATP depletion (Szabo et al., 1996). Considering then that NAD is essential for oxidative phosphorylation and the resynthesis of ATP, it can be understood how cells which are in that situation are not capable of maintaining their ionic equilibria and can undergo degeneration both of necrotic and of apoptotic type. Excessive activation of PARP under the action of newly formed nitric oxide can occur in the central nervous system following activation of the glutamate receptors of the NMDA type associated with neuronal nitric oxide synthase (nNOS). It can also occur in other organs or tissues, since nitric oxide can be synthesized by the NOS of the endothelium or by the inducible NOS of the microglia and macrophages, and diffuse into the surrounding cells. Much experimental data seems to be in agreement with this hypothesis and it has been clearly demonstrated that by inhibiting PARP it is possible to reduce excitotoxic neuronal death or that due to free radicals in various types of neuronal culture (Schraufstatter et al., 1986; Cosi et al., 1994; Zhang et al., 1994). It has also been shown that baby mice with this enzyme knocked out are particularly resistant to cerebral tissue loss after occlusion of the medial cerebral artery or endocrine cell loss after administration of toxins. It has also been shown that cells taken from such mice are very resistant to oxidative stress (Eliasson et al., 1997; Endres et al., 1997; Pieper et al., 1999).
Thus the importance of having available pharmaceutically acceptable PARP inhibitors is clear.